1. Field of the Invention
The invention relates to a process for the preparation of 2-methylene aldehydes by reacting aldehydes having at least two carbon atoms on the .alpha.-carbon atom with formaldehyde in the presence of a catalyst system consisting of a mixture of a secondary amine and an organic carboxylic acid.
2. Discussion of Prior Art
2-Methylene aldehydes (.alpha.-methylene aldehydes, .alpha.-alkylacroleins) can be prepared in a number of ways. For example, the reaction of ammonia or a primary or secondary amine, usually present as a salt such as hydrochloride, with formaldehyde and a compound containing a reactive hydrogen atom, generally known as the Mannich reaction, will yield the desired methylene compounds.
In accordance with the process described in U.S. Pat. No. 2,518,416, a mixture of an aldehyde which has a CH.sub.2 group in the .alpha. position relative to the carbonyl group and of formaldehyde is passed through the melt of a salt formed of a primary or secondary amine and a strong acid.
In accordance with the process described in U.S. Pat. No. 2,639,295, the condensation of aliphatic aldehydes with formaldehyde is carried out in the presence of a piperidine hydrochloride, morpholine hydrochloride or an ammonium salt such as ammonium chloride.
One characteristic which the processes cited above have in common is that condensation takes place in the presence of salts of the amines or of the ammonia, which must be used in a stoichiometric amount or even in excess.
While in German patent No. 16 18 528 it is pointed out that .alpha.-methylene aldehydes can be obtained by reaction of aldehydes of the general formula RCH.sub.2 --CHO with formaldehyde in the presence of catalytic amounts of a primary or secondary amine, it is apparent that in all examples illustrating the process claimed the amine is always used in the form of its salt, and always in amounts greater than what might be regarded as catalytic amounts.
The requirement that the condensation of aldehydes which contain a CH.sub.2 group in the .alpha. position relative to the carbonyl group with formaldehyde be carried out in the presence of large amounts of an amine militates against economic utilization of the reaction. Moreover, working with amine salts such as hydrochloride calls for the use of equipment made of high-alloy special steels or of enameled reactors if damage to the reactors due to stress-corrosion cracking, for example, is to be avoided.
Finally, in these prior-art processes conversion, selectivity and yield are unsatisfactory, which imposes a limit on their practical use.
Thus, there has been a need for a process for the preparation of 2-methylene aldehydes which does not have the drawbacks outlined and in which the starting materials are converted to the desired products through a readily performed reaction in high yield.
In accordance with U.S. patent application Ser. No. 106,211, assigned to the assignee hereof, the disclosure of which is hereby specifically incorporated herein by reference, catalysts of the same composition are used solely for the preparation of methacrolein. But while it was originally through that said process would lend itself only to the preparation of methacrolein, it has now surprisingly been found that the process in accordance with the invention can be used for the preparation of 2-methylene aldehydes generally by reaction of aldehydes of the formula R--CH.sub.2 --CHO with formaldehyde if the catalyst consists essentially of 0.01 to 0.05 mols of a secondary amine and 0.005 to 0.02 mol of the carboxylic acid, both molar amounts based on the aldehyde of the formula R--CH.sub.2 --CHO.